A long established trend in mobile communications is to squeeze increasing amounts of data over a fixed bandwidth. Voice and multimedia services (i.e.: session initiation and description, real-time transport, and eventually hypertext transport and real-time streaming video) are now available to users of third generation (3G) mobile devices through Internet Protocol Multimedia Subsystem (IMS) architecture. IMS is specified by the industry consortium Third Generation Partnership Project (3GPP) as part of the Global System for Mobile Communication/Enhanced Data Rates for Global Evolution (GSM/EDGE or GERAN) standard. IMS has the potential to greatly increase the demand for higher volumes of data flowing over GERAN.
Mobile systems such as those employing the GSM standard facilitate transmissions in quanta of ‘frames’, wherein the frame size and time are fixed but the data rate within a frame may vary. Each transmission frame is divided into eight multiplexed channels. For full rate (FR) channels, one slot every frame is used. For half rate (HR) channels, one slot every second frame is used. Each TDMA frame allows 8 full rate users or 16 half rate users to be multiplexed. Switching between FR and HR occurs frequently, and allows bandwidth to be used efficiently. For example, when the volume of transmissions within the ‘cell’ of a base station is high, the base station may convert certain transmissions to HR channels in order that the wireless connections of all current users remain uninterrupted. Once traffic volume within the cell subsides, the base station may then allow full rate transmissions for all current users. HR channels may be switched to FR channels when a user is moving between cells and the connection is being handed over from one base station to another. Additionally, data-intensive portions of a wireless transmission, such as high quality voice transmissions or graphical interface downloads, may require FR channels. Other less data-intensive portions may be switched automatically to HR channels without any loss in quality.
Data flows between the physical layer (layer 1) and the medium access control layer (MAC or layer 2) along logical channels, which are divided into traffic channels and control channels. Logical channels are multiplexed by the MAC on physical subchannels, which are units of the radio medium. Traffic channels of the type TCH are intended to carry encoded speech and user data, while control channels carry signaling and synchronization data. The TCH use a circuit-like connection over the radio interface, and can be modulated either by Gaussian Minimum Phase Shifting (GMSK) or 8-Phase Shift Keying (8-PSK). One main characteristic of the channel coding for the TCH is the use of diagonal interleaving.
Both traffic and control channels may change between half rate and full rate channels. Any time there is a change, both the base station and the mobile station must do so quickly and in concert with one another in order to ensure bandwidth is available where it is needed (i.e.: graphical or streaming downlinks, new users in a cell initiating a cellular connection) and not wasted (i.e.: using full rate channels for typical voice transmissions). Switching between full and half rate entails a risk of dropped or delayed connections, which must be kept to a minimum. There is a current proposal to employ a flexible layer overlay (Flexible Layer One, or FLO) in release 6 of GERAN that is to be used in conjunction with the physical layer to interface with the medium access control (see 3GPP TR 45.902, herein incorporated in its entirety). However, because the interleaving diagonal depth differs between full rate and half rate channels, the proposed standard is not compatible with half rate signaling. What is needed in the art is an improved method of switching between full rate and half rate transmissions, especially one consistent with the GERAN standard to better enable IMS. It is preferable that such an improved method operate within the proposed FLO rather than require an entirely new standard. The present invention is directed to such an improved method. Further background may be found at 3GPP TS 43.051 (“Overall Description—Stage 2”) and 3GPP TS 45.003 (“Channel Coding”), each from GERAN release 4.